The present invention relates to a continuous furnace for the heat-treatment of workpieces comprising an endless chain-like conveyer belt consisting of a ceramic material and having a plurality of links which are arranged one beside the other and one behind the other and which are interconnected by rods extending transversely to the feeding direction.
A continuous furnace of the describe type, which has a conveyer belt consisting of a ceramic material, has been known, for example, from a leaflet published by CERAMIC FURNACE BELT Inc., U.S.A.
The continuous furnace comprises a heating and a cooling zone, and the endless conveyer enters the continuous furnace by its inlet end, passes both the hot and the cold zones of the furnace, and leaves the furnace by its run-out end. Thereafter, the conveyer belt is returned from the run-out end, via a guide pulley, underneath the furnace and then back to the inlet end via another guide pulley.
The conveyer belt as such consists of a plurality of links arranged one beside the other which, viewed in transverse direction, are interconnected by a single rod extending over the full width of the belt. The individual links, and the rods as well, consist of a ceramic material, for example of aluminium oxide. Conveyer belts consisting of ceramic materials have the advantage, as compared with metallic belts, that they are lighter in weight, can be exposed to higher temperatures and are, in addition, more resistant chemically.
A ceramic conveyer belt of this type is exposed continuously to heavy temperature variations and is, thus, subjected to very high and varying thermal stresses. In addition, considerable mechanical stresses are exerted upon the belt, the latter having a considerable weight of its own when the furnace has a length of 20 m for example, so that the prevailing frictional forces are relatively important and correspondingly high mechanical traction forces have to be applied for moving the belt.
On their way through the system, the chain links as well as a rod connecting them in the transverse direction are exposed to varying temperatures between approximately 1.300.degree. C. and 20.degree. C. so that significant internal stresses occur in the material, in spite of the latter's relatively low coefficient of expansion. It is a drawback of such a conveyer belt comprising rods extending over its full width that the stresses developing over the entire width, due to the varying thermal strains and the uninterrupted mechanical stresses, lead to frequent breakage of the rods. This may even lead to the conveyer belt breaking apart fully in the area of a transverse rod, an event which, when occurring in the hot zone, results in extensive dismantling work.